"A flashy feature that has limited usability for victory."
In TheMasksWeWear, both Zuko and Azula learn how to use their firebending to fly by using controlled jets of flame.
In the Uplifted series, The Tiger tank prototype, as Hoch and Hanala find out when trying to take one deep behind enemy lines in Libya. It's clunky, over engineered, and unreliable in the conditions they face.
In Aeon Natum Engel and Aeon Entelechy Evangelion the Military views the Evangelions as unnecessary large Engel-prototype relics from the past. In the latter this view is somewhat extended to the conventional mecha, at least by the tank crews.
BlackMegaGargomon from the Tamers Forever Series has an unbelievably powerful technique known as the Black Moon Flare, which causes unfathomable amounts of devastation within a vast radius. Unfortuneatly, the technique happens to be a little too powerful, and D.C is only ever able to manage one use of it. And that ends up being a Worf Barrage
In Calvin At Camp, Calvin, Jason, and Marcus try to attack Kevin with medieval weapons, but find out the hard way that although the weapons look cool, they are impossible to lift.
In Shadow Snark, the titular characters wings are made of solid iron, which though badass doesn't give him the ability to fly. A very crippling handicap for a pegasus.
In Princess of the Blacks, Jennifer (A Gender Flip of Harry) uses a form of invisibility that's undetectable to Moody's magic eye. However, since it works by bending light away from a person, they're likewise blind for its duration. The only reason Jennifer uses it is because she's already blind (though on a potion regimen to regain her sight).
In the My Little Pony: Friendship Is Magic fanfic, The Dusk Guard: Rise resident Wrench Wench Sky Bolt has several ideas for things like fast and powerful airships and protective armor, but acknowledges that they are too expensive and material-heavy for mass production and general use. For a six-pony Elite Army such as the Dusk Guard however, they're absolutely perfect. In fact, it is the ability to pursue these ideas that Steel Song uses to entice Sky Bolt into the unit.
In Bleach/Naruto crossover True Warriors Never Die, Naruto is the jinchuuriki of the Juubi which (in addition to having 2000 years to hone his skills) gives him destructive power comparable to Yamamato. The downside is that he has to fight the Juubi roughly every 1000 years to insure it doesn't try to destroy the world again. As he recently found out, the more he uses its power, the faster the Juubi recovers. In fact, despite using his jinchuuriki mode only a few times, the Juubi is already stirring, 900 years early.